First of all: what is the "mass"?

We know that a photon has no-mass (ne rest mass): so we can say that if ipotetically we can measure something about a photon (the same photon) in each region of the universe we must notice that it is everywhere at the same time: no mass means no time evolution.
And no mass means "spread it on the whole universe".

But we know also that the QFT says that each body, each entity has the behaviour of light: it can be revealed everywhere.... not really with the same probability as the light, but (for example) an electron could be here, or there, or near Alpha Centauri.

The problem of the "inertia" is strictly related to "the mass of the entity, of the subject of inertia... and the mass of the rest".

Knowing now that it is just a "ficticious" representation, a mental image, our subdivision of the universe in different bodies... we conclude that all the universe is only 1 thing, only 1 entity... that appears to us as modelled in many many different entities that interact through the
cause-effect paradigm.

If we look deeply at the Einstein eq.ns we see that all the energies/masses of the system (in our case of the universe) are subject to a definite rule that arises as a whole from the beginning... and has no time evolution.

So:
.... on the cause of inertia.
Is it caused by mass in the universe?
Is it a property it derives from within itself?
true the 1st true the 2nd

Is there a time delay before it starts?
Does it last for a time after acceleration stops?

hmmmmmm.... I do not know: does the Graviton exist?????